oAuth is an option that is offered based on the server name. Or it is auto filled when the the new account wizard does it's thing. So in what circumstance are you saying it is not an option? If you are changing the server name, make the authentication normal password and exit the setting page. Open the settings again and change the authentication method to oauth. However why would you be trying to manually change the authentication method on a gmail account? Every existing account was automatically upgraded to oauth around a year ago and all new accounts use it by default.